1. Field of the Invention
The present invention relates to the field of signal equalization within wavelength division multiplexed optical transmission networks. More particularly, the present invention relates to the use of diffractive light modulators for signal equalization of component signals within wavelength division multiplexed optical fiber networks.
2. Background of the Invention
Increasingly, data transmitted through the telecommunications network is shifting from an electrical data transmission to optical data transmission. Design parameters for fiber optical networks seek to create an architecture which allows the highest number of bits per second in transmission, while simultaneously reducing costs by developing a system that affords the greatest distance between repeaters, and maintaining system reliability where transmission errors are held to an acceptably low level. These three operational characteristics are oftentimes adverse to each other. Signal strength, that is, the power of the signal, attenuates during transmission, and the greater the transmission distance, the greater the attenuation. As signal strength decreases, the signal to noise ratio decreases. The bit error rate increases exponentially as the signal to noise ratio decreases. The need for repeaters is therefore governed largely because of line loss or attenuation of an optical signal through an optical medium. At some critical point in attenuation, a signal under transmission will become be too weak to reproduce reliably, and the bit error rate will climb to unacceptable levels. Accordingly, a lower threshold for signal strength is established as a system parameter of an optical network to ensure system reliability. To operate within such a parameter, the system must be designed to prevent a signal from falling below the lower threshold. Typically, signal attenuation is measured in dB per km. Accordingly, repeaters must be spaced close enough together that a signal has not attenuated below the lower threshold by the time it reaches the next repeater, at which point it is either amplified, or processed in some other manner for re-transmission. Because it is economically dis-advantageous to have repeaters spaced more closely together than necessary, repeaters are typically placed near the maximum distance at which a signal can be reliably received and processed for accurate reconstruction.
In endeavoring to maximize the distance between repeaters, an increase in the power of a signal will therefore increase the distance that a signal may reliably travel before falling below the lower threshold. It is easily understood, however, that there is an upper limit of allowable power when transmitting a signal across an optical network. Above that upper limit, an increase in signal strength is at best superfluous, and at worst, maintenance intensive, economically prohibitive or even deleterious to the integrity of the optical network. The upper threshold of signal transmission power is therefore another operational parameter for a fiber optical network.
Analog and digital communication have long used frequency/wavelength multiplexing as one means of achieving greater bandwidth. Through multiplexing, discrete signals defined by distinct wavelengths are transmitted across the same medium. Each discrete signal is typically assigned to carry specific information. Signal attenuation within a fiber network, however, is oftentimes frequency or wavelength dependent. Accordingly, the rate of attenuation, commonly measured in dB/km can vary among different wavelengths within a fixed optical spectrum. Consequently, the wavelength distinguished by the highest rate of attenuation will typically govern fundamental network parameters such as the maximum distance between repeaters. One result of differing rates of attenuation, therefore, is that different wavelengths transmitted at a same power will be at different power levels upon reaching a repeater or other processing station. In addition, the routing and switching of signals within a metropolitan network has the capacity to combine signals of disparate power levels. Moreover, there is unevenness in the multiplexing and demultiplexing components, unequal gain over different wavelengths in erbium doped fiber amplifiers (EDFAs), unequal laser launch power for the different channels, etc. All of these features exacerbate the uneven power levels of different wavelengths during the transmission, re-transmission, routing and processing of an optical signal.
FIG. 1 illustrates a spectrum made up of many discrete wavelengths, from a first wavelength xcex1 up to an nth wavelength xcexn, which form component signals within a collective wavelength multiplexed signal within an optical medium. The Y-axis represents signal power, and the X-axis represents a spectrum of wavelengths. It is commonly understood by those skilled in the art that wavelength and frequency are inversely proportional. These terms may therefore be used interchangeably throughout to distinguish component signals. Moving along the X-axis is therefore equally understood to represent a spectrum of frequencies. The lower signal threshold 124 is the lowest signal power level to which a signal may attenuate and remain reliably processable according to system requirements. The xe2x80x9csaturation thresholdxe2x80x9d 120 is the maximum allowable signal power of the network for any one wavelength. Between these two levels, a reference power level 122 is illustrated throughout FIGS. 1, 2 and 4 for comparative purposes only. For illustrative purposes, it is assumed that all of the component wavelengths or frequencies depicted in FIG. 1 began at equal signal strength, and have attenuated to the levels seen in FIG. 1 during launch, transmission, routing or other processing within a fiber optical network. As seen in FIG. 1, the signals can be at different strengths. The third wavelength xcex3 is seen to be quite robust, remaining above the reference level 122. Contrariwise, the fourth wavelength xcex4 is seen to have attenuated to a signal strength substantially below the reference level 122.
FIGS. 2 and 3 show the signals of FIG. 1 after each component wavelength has been uniformly amplified. Because the third wavelength xcex3 was the strongest signal prior to amplification, it remains the strongest signal after amplification. Plotting uniformly amplified signals, the relationship in signal strength is therefore unchanged from the pre-amplification relationship of FIG. 1, provided all of the component signals remain below the saturation threshold. FIG. 2 shows all component signals within the upper limit of the network parameters, with the strongest signal, the third wavelength, xcex3, at the upper limit. As noted however, the other discrete wavelengths fall far below the upper threshold. Because it was earlier determined that the fourth wavelength xcex4 was subject to the greatest attenuation during transmission, future transmission subsequent FIG. 2 is limited by the fourth wavelength xcex4, which is both the weakest signal, and subject to the greatest attenuation. Failure to amplify the fourth wavelength xcex4 to the maximum allowable signal strength 120 will result in attenuation of xcex4 to the lower threshold 124 in a substantially shorter transmission distance than if it had begun at the upper threshold 120. Alternatively, FIG. 3 shows the fourth xcex4, which is the weakest component wavelength in the figure, amplified to the upper threshold 120. The problem with this approach, however, becomes clear when an examination is made of the other component signals in FIG. 3. By amplifying the weakest signal up to the upper threshold 120 of the network, in a uniform amplification process, all other signals, xcex1, xcex2, xcex3, xcexn are amplified above the upper threshold 120 of the optical network.
To optimize network performance therefore, a first step in the processing of a wavelength multiplexed signal is channel equalization of component signals xcex1, . . . , xcexn. FIG. 4 illustrates component signals in a wavelength multiplexed signal which have been both equalized, and amplified to the upper threshold 120 of the network parameters. Unless the weakest component signals xcex1, . . . , xcexn is below the allowable threshold for maintaining an acceptable signal to noise ratio, the first step of the equalization process is to reduce the signal strength of each component wavelengths component signals xcex1, . . . , xcexn to the level of the lowest power of any of the signals present. Alternatively, the component signals may be reduced to a common predetermined power level. The second step in the equalization process is to uniformly amplify the equalized component signals xcex1, . . . , xcexn to a predetermined power level, preferably the maximum recommended power level 120 (FIG. 4) of a network. By this process of equalization and amplification, all wavelengths within a signal can be equally amplified to the maximum power allowable on a fiber network, thereby maximizing the signal to noise ratio and minimizing the bit error rate.
Initially, optical signal equalization was performed electrically by converting component optical signals into electrical signals, amplifying the component signals and converting the signals back into optical signals. The process, often known as regeneration, typically included a variety of drawbacks. The process required an optical receiver, an electrical amplifier, and an optical transmitter. Moreover, the repeaters or regenerators were typically monochromatic, requiring a different set of components for each wavelength or channel. Accordingly, the process was complex and expensive to maintain. As a result, systems have increasingly looked to optical equalization in wavelength division multiplexing systems. Methods of optically equalizing and amplifying signals have included static equalization and dynamic equalization. The use of an erbium doped fiber amplifiers (xe2x80x9cEDFAsxe2x80x9d) combined with a dielectric filter, whose transmission is, for example, the spectral inverse of the gain spectrum of an erbium doped fiber amplifier (xe2x80x9cEDFAxe2x80x9d), can function as a static equalizer. The EDFA and filter taken together then work to produce a flat, equal output spectrum. However, such an approach is largely incapable of dynamic equalization, that is, equalizing the spectrum under changing conditions. The spectral variation of gain for an EDFA can be 2-3 dB or, at the extreme spectral edges of the amplifier, as large as 15 dB. Furthermore, the gain curve of an EDFA changes dynamically as the input power levels on the individual wavelength channels change. Such dynamic deviations in the EDFA gain curve cannot be corrected with a dielectric filter. However, equalization is highly desirable if the signal to noise ratio (xe2x80x9cSNRxe2x80x9d) values are also to be maximized for all channels in a WDM system.
Because of the limitations of static equalization, engineers have sought to develop a reliable dynamic means of equalization. A dynamic equalizer would allow a more complete use of EDFA gain with a consequent reduction in the number of EDFAs in a given transmission distance. A variety of dynamic equalization techniques have also been advanced within the prior art, which seek to equalize component signals in a WDM system. All rely on some spectral mux/demux component, followed by an electronically-controllable variable optical attenuator (xe2x80x9cVOAxe2x80x9d) which can operate on the de-multiplexed channels (or possibly a band of channels).
Mach-Zehnder interferometers, which have been well-established as effective high-speed amplitude modulators, can be used as high-resolution VOA""s. The Mach-Zehnder thermo-optic filter functions as a temperature-controlled waveguide interferometer. An optical path length may be further controlled by changing the temperature of the refractive material in the path. The amplitude of two different channels or wavelengths entering a directional coupler are split equally among separate paths. The path length is thereby controlled. The beams are recombined at a second direction coupler with two different outputs. Each output typically supports only one of the wavelengths under certain constructive phase conditions. Accordingly, different wavelengths can be tuned through optical path differences by controlling the temperature of the refractive material. However, Thermo-optical apparatus typically dissipate considerable amounts of heat into the substrate and are inherently slow.
Another means for dynamic equalization being explored is through the use of semiconductor optical amplifiers (SOAs). SOAs create a gain medium through population inversion by electrically pumping a semiconductor, for example, indium gallium arsenide phosphide (InGaAsP). The weaker input WDM signals optically seed the gain medium and are amplified through stimulated emission. However, although the spectral bandwidth that can be addressed by SOAs is fairly large, SOAs have a low signal-to-noise ratio, suffer from significant channel cross-talk, and are polarization sensitive, typically requiring polarizing-preserving fibers for transmission.
Raman amplifiers have also been used for dynamic optical equalization. Raman amplifiers use a lower-wavelength pump laser to excite the atoms in nondoped fibers to higher energy states. The amplification created by this process is not linear, but rather, the weaker WDM signals have a greater effect of stimulating atoms in higher energy states to emit photons at longer wavelength commensurate with the WDM signals. This non-linearity can be exploited to dynamically equalize component WDM signals of varying power levels. The stimulated light mixes with the WDM signal, resulting in optical amplification. The spectral bandwidth over which Raman amplifiers operate is fairly wide. However, Raman amplifiers suffer from their own limitations and drawbacks. For example, Raman amplifiers require very long fibers and high-powered pump lasers. Other approaches include acousto-optic tunable filters (xe2x80x9cAOTFsxe2x80x9d), which have been employed as spectroscopic attenuators for years.
A more recent technology used in dynamic equalization of WDM signals has been electronically switchable Bragg gratings (ESBGs). ESBGs can be created through holographic polymer-dispersed liquid-crystal technology (HPDLC), which embeds phase-volume holograms in polymer substrates through a process that allows direct control of the diffractive bandwidth and central wavelength. The liquid-crystal droplets comprising the ESBG are therefore placed on a waveguide. The components are made by creating a row of ESBGs, wherein each ESBG is devoted to a predetermined wavelength. The ESBGs are formed of a mixture of liquid crystal and polymer, which exhibit Bragg Gratings, or a series of stripes of different refractive index, disposed within them to reflect back specific wavelengths. An interference pattern is formed by exposing the polymer and liquid crystal mixture to ultraviolet light from intersecting laser beams. Liquid crystal (LC) droplets are formed as the liquid crystal diffuses to areas of high light intensity. When a voltage is applied to this arrangement, the refractive index of the LC droplets is reduced, thereby collapsing the grating effect and allowing all light to pass through. When no voltage is applied, the grating diffracts light at a specific wavelength of the waveguide. Accordingly, by applying an appropriate voltage, the LC droplets form an evanescently coupled grating which can couple light out of the waveguide to a varying level, depending on the coupling-strength of the grating.
In processing a WDM signal, however, it is generally advantageous to process component wavelengths in parallel rather than serial processes. In parallel processing of component wavelengths, all wavelengths are processed simultaneously, so there is no cumulative signal attenuation or degradation. In serial processing of the component wavelengths in a WDM signal, those wavelengths that are not processed first may be subject to cumulative attenuation or degradation imposed in the successive processing steps. According to such a serial process, if four wavelengths are being equalized, the first wavelength is processed at the first serial station, and the second, third and fourth wavelengths are subjected to any attenuation, scattering, filtering, refraction or other degradation imposed by the first station. When the second wavelength is processed by the second station, the third and fourth wavelengths are subject to any degradation imposed by the second station. When the third wavelength is processed at a third station, the fourth wavelength is again unnecessarily degraded by the interaction at the third station. Accordingly, if serial WDM imposes any degradation whatsoever upon wavelengths awaiting processing, the later processed signals will degrade exponentially according to the number of serial processing steps or stations, thereby limiting the scalability of such a technique, and placing an upper limit on the number of wavelengths that may be processed.
Preliminary considerations suggest that a WDM signal being processed through a liquid crystal ESBG advantageously directs the WDM signal through a succession of liquid crystal devices in a serial application, each device dedicated to a single wavelength. According to this model, a WDM signal being serially processed by liquid crystal ESBGs will cascade through a series of liquid crystal device, thereby limiting the scalability or the total number of channels which may be processed in this manner. Although studies continue to advance on the functionality and application of ESBG technology, its applications, and limitations, both commercial and technological, remain largely unexplored.
There exists therefore a need for a method and apparatus of equalizing an optical signal without converting the optical signal to an electrical signal and back to an optical signal. There is also a need for a method and apparatus which can dynamically equalize an optical signal as the power levels on the individual wavelength channels change, thereby responding to dynamic deviations in a EDFA gain curve. There further exists a need for a method and apparatus for dynamically equalizing optical signals that is very fast, has low insertion loss, does not require the use of very long fibers and high-powered pump lasers, that exhibits a low signal-to-noise ratio, does not suffer from significant channel cross-talk, and is not highly polarization sensitive such that it requires polarizing-preserving fibers for transmission, and is premised on economically viable and commercially proven technology which is demonstrated to avoid the limitations of serial processing of light waves.
The present invention discloses a method and apparatus for equalizing the power levels of component wavelengths within a wavelength multiplexed signal, without the need of converting the optical signal to an electrical signal and back to an optical signal. The present invention further discloses a method and apparatus which can dynamically equalize an optical signal as the power levels on the individual wavelength channels change, thereby responding to dynamic deviations in a EDFA gain curve. The present invention further discloses a method and apparatus for dynamically equalizing optical signals that is very fast, does not dissipate substantial heat to the substrate, does not require the use of very long fibers and high-powered pump lasers, that exhibits a low signal-to-noise ratio, does not suffer from significant channel cross-talk, and is not highly polarization sensitive such that it requires polarizing-preserving fibers for transmission. The present invention further discloses a method and apparatus for dynamically equalizing a WDM signal that is premised on economically viable and commercially proven technology which is demonstrated to avoid the limitations of serial processing of light waves. These and other advantages will become apparent to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawings and figures.
According to one embodiment of the present invention, an apparatus for adjusting power levels of component signals of a wavelength division multiplexed signal including a first wavelength signal and a second wavelength signal, comprises a diffractive light modulator with a first pixel configured to receive the first wavelength signal and a second pixel configured to receive the second wavelength signal. The first pixel directs at least a portion of the first wavelength signal into a first mode, whereby a first post-modulator wavelength signal is formed, a portion of the first post-modulator wavelength signal being collected in a first optical output channel to form a first collected signal. The second pixel partially directs the second wavelength signal into the first mode and partially directs the second wavelength signal into a second mode, whereby a portion of a second post-modulator wavelength signal is collected in a second optical output channel to form a second collected signal. A power level of the second collected signal is attenuated relative to a power level of the second wavelength signal.
The apparatus advantageously comprises a demultiplexer configured to de-multiplex the wavelength division multiplexed signal into component signals including the first wavelength signal and the second wavelength signal, and a multiplexer configured to multiplex a plurality of collected signals into an output signal, the plurality of collected signals including the first collected signal and the second collected signal. The apparatus is configured such that the power level of the first collected signal is approximately equal to the power level of the second collected signal. The apparatus advantageously comprises a first light sensor for determining a power level of the first collected signal and a second light sensor for determining the power level of the second collected signal. A controller is electrically coupled to the first light sensor, the second light sensor, and the diffractive light modulator. As the first and second light sensors detect changes in the first and second wavelength powers, the controller dynamically modulates the first and second pixels of the diffractive light modulator to maintain a desired level of attenuation, maintaining a predetermined ratio between the power level of the first collected signal and the power level of the second collected signal. According to one embodiment, the first wavelength signal passes through the first lens before coupling with the first pixel, and the second wavelength signal passes through the second lens before coupling with the second pixel. The diffractive light modulator advantageously comprises a grating light valve.
A method of adjusting a power level of component wavelength signals of a wavelength division multiplexed signal comprises the steps of de-multiplexing an input signal into component wavelength signals including first and second wavelength signals, illuminating first and second pixels of a diffractive light modulator with the first and second wavelength signals, respectively forming first and second post modulator signals, modulating the first pixel such that a portion of the first post-modulator signal is directed into a first optical channel, thereby forming a first collected signal, wherein a portion of the first post-modulator signal is directed away from the optical channel, thereby forming a first rejected signal. A reference power level is established, wherein the first collected signal is modulated to conform to the reference power level. The modulating is achieved by modulating of the first pixel through a controller. According to one embodiment, the step of establishing a reference power level comprises the steps of determining a plurality of power levels respectively associated with the plurality of collected signals, and selecting a reference power level from among the plurality of power levels. According to an alternative embodiment, the reference power level is predetermined.
A channel equalizer for a wavelength division multiplexing system comprises an optical input for transmitting an input signal, the input signal comprising a first plurality of component input signals defined according to a first plurality of wavelengths. The optical input is operatively coupled to a demultiplexer which is configured to separate the input signal into the first plurality of component input signals including a first component input signal defined according to a first wavelength. A diffractive light modulator comprises a plurality of pixels including a first pixel. A plurality of input channels respectively channel the plurality of component input signals from the demultiplexer to the respective plurality of pixels, including a first input channel configured to receive the first component input signal and channel it toward the first pixel of the diffractive light modulator. A plurality of reflected signals are formed by the plurality of component input signals interacting with their respective pixels. The plurality of reflected signals are directed toward a plurality of output channels, including a first reflected signal reflected toward a first output channel. The first output channel is configured to receive a portion of the first reflected signal, the portion ranging from zero to one hundred percent, thereby forming a first collected signal. The first pixel is configured to controllably modulate the portion of the first signal received by the first output channel. A multiplexer is operatively coupled to the plurality of output channels. The multiplexer is configured to receive a plurality of collected signals including the first collected signal, the multiplexer being further configured to combine the plurality of collected signals into an output signal. The channel equalizer is advantageously coupled to a plurality of light sensors for measuring the power level of a plurality of signals, including a first light sensor for measuring the power level of a first component wavelength. According to one embodiment, the first light sensor is coupled to the first input channel for measuring the power level of the first collected signal. According to this embodiment, the first light sensor may measure the power level of the wavelength of light prior to being multiplexed into a single stream, or may measure the power level of the first component wavelength after a plurality of components have been multiplexed into an output stream, wherein a portion of the output signal is diverted, demodulated into its component wavelengths for measuring by a plurality of light sensors. Alternative embodiments are envisioned, however, wherein the first light sensor measures the power level of light thrown away from the first output channel as a result of the diffractive light modulator prior to entering an output channel. Still another embodiment is envisioned wherein the first light sensor measures the power level of the first component wavelength prior to striking the diffraction light modulator. The diffractive light modulator is preferably a grating light valve.
A method of selectively equalizing respective power levels of a plurality of component signals defined by a plurality of wavelengths comprises the steps of directing the plurality of component signals onto a respective plurality of pixels of a diffractive light modulator, wherein a first component signal is directed onto a first pixel, and wherein a plurality of signals resulting from an interaction of the plurality of component signals with the plurality of pixels are defined as a plurality of reflected signals, including a first reflected signal resulting from the first component signal interacting with the first pixel, controlling the first pixel by a controller to affect an amount of diffraction created within the first reflected signal, and collecting a portion of the plurality of reflected signals in a respective plurality of optical output channels, thereby forming a plurality of collected signals, including a first collected signal comprising a portion of the first reflected signal entering a first optical output channel. The method advantageously comprises the step of measuring a portion of the first reflected signal to determine a power level of the collected portion of the first reflected signal. According to one embodiment, the measured portion of the first reflected signal comprises the collected portion of the first reflected signal. According to an alternative embodiment, the measured portion of the first reflected signal comprises an uncollected portion of the first reflected signal.